Optical waveguide device and method of manufacturing the same

ABSTRACT

An optical waveguide device includes a substrate; a lower cladding disposed on the substrate; a rib waveguide including a slab disposed on the lower cladding and a single rib disposed on the slab contiguous to the slab; and an upper cladding disposed on the rib waveguide. The rib waveguide includes a first doped region having a first electric conductivity exhibiting a P-type electric conductivity across the rib and the slab and a second doped region being contiguous to the first doped region and having a second electric conductivity exhibiting an N-type electric conductivity across the rib and the slab.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application based on a PCT PatentApplication No. PCT/JP2014/060558, filed Apr. 7, 2014.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical waveguide device thatenables high-speed refractive index modulation with low optical loss andlow driving voltage and a method of manufacturing the optical waveguidedevice.

Description of the Related Art

In recent years, the application of small-footprint optical integratedcircuits such as planar optical waveguides to optical-fibercommunication devices, in particularly, to optical transport equipmentused for long-haul or metro-area wavelength-division multiplexingoptical-fiber communication and optical interconnect equipment used indata centers and the like has been studied.

U.S. Pat. No. 7,085,443 (hereinafter referred to as PTL 1) discloses asingle-mode silicon rib waveguide, in which refractive index iscontrolled by changing carrier density in PN junction formed in thetransverse direction and the position of the PN junction boundarychanges in a corrugated form along a direction of light propagation inthe core of the optical waveguide.

Zhi-Yong Li, Dan-Xia Xu, W. Ross McKinnon, Siegfried Janz, Jens H.Schmid, Pavel Cheben and Jin-Zhong Yu, “Silicon waveguide modulatorbased on carrier depletion in periodically interleaved PN junctions”,Optics Express vol. 17, no. 18, pp. 15947-15958 (2009) (hereinafterreferred to as NPL 1) discloses technical information about the opticalcharacteristics of a silicon waveguide in a configuration similar tothat of the silicon rib waveguide of PTL 1. NPL 1 reports that, in theconfiguration disclosed in PTL 1 and in an optical modulator consistingof a silicon optical waveguide having a similar configuration to theabove-described configuration, Bragg reflection caused by the periodicprofile of the refractive index is negligible. Furthermore, NPL 1discloses design exemplification of an optical modulator operating at alonger wavelength than the Bragg wavelength.

United States Patent Application, Publication No. 2012/0189239(hereinafter referred to as PTL 2) discloses an optical modulator inwhich optical loss is reduced by using a configuration in which thedepletion region in the PN junction is sandwiched between the first andsecond intrinsic regions. The first and second intrinsic regions aredisposed contiguously to respective one of the two opposing side wallsof the optical waveguide.

In the silicon rib waveguide disclosed in PTL 1 in which the position ofthe PN junction boundary changes in a corrugated line along a directionof the light propagation in the core of the optical waveguide, since theeffective length of the PN junction increases, the driving voltage isreduced. However, in this case, there is a problem of an increase inoptical loss due to an increase of optical absorption by carriers. Inaddition, there is another problem that it is not possible to reduceparasitic capacitance caused by the fringe electric fields from slabregions existing in side sections of the rib waveguide and high-speedoperation is impaired. Therefore, in a case in which the above-describedsilicon rib waveguide is used, high-speed refractive index modulationwith low optical loss is difficult.

In a design based on NPL 1, when a rib waveguide having a corrugateddistribution profile in high refractive index contrast is used, returnloss is decreased due to Bragg reflection and optical feedback to alaser light source generating incident light to the optical modulator issignificant and mode hopping is generated, whereby the laser lightsource becomes unstable. As a result, there is a problem that it is notpossible to generate stabilized optical modulation signals.

In the optical modulator disclosed in PTL 2, when the driving voltage isreduced, the refractive index modulation degrades. Therefore, there is aproblem that it is not possible to reduce the driving voltage.Furthermore, in PTL 2, there are additional problems that the influenceof fabrication error is significant and it is not possible to provide anoptical modulator with small quality variation.

As described above, an object of the invention is to realize an opticalwaveguide device that is suitable for small-footprint optical integratedcircuits such as an optical modulator enabling high-speed refractiveindex modulation with low optical loss and low driving voltage andallows only small quality variations for application to optical-fibercommunication devices, in particularly, to optical transport equipmentused for long-haul or metro-area wavelength-division multiplexingoptical-fiber communication and optical-interconnect equipment used indata centers and the like.

SUMMARY

An optical waveguide device according to a first aspect of the presentinvention includes a substrate; a lower cladding disposed on thesubstrate; a rib waveguide including a slab disposed on the lowercladding and a single rib disposed on the slab contiguous to the slab;and an upper cladding disposed on the rib waveguide, wherein the ribwaveguide includes a first doped region having a first electricconductivity exhibiting a P-type electric conductivity across the riband the slab and a second doped region being contiguous to the firstdoped region and having a second electric conductivity exhibiting anN-type electric conductivity across the rib and the slab, a boundarybetween the first doped region and the second doped region provides a PNjunction formed in a direction perpendicular to a surface of thesubstrate and is disposed in a corrugated line in a propagatingdirection of guided light in the rib waveguide in a plan view of thesubstrate, and the rib waveguide includes at least one of a first lowconductive region being contiguous to an opposite side of the seconddoped region in the rib and exhibiting lower electric conductivity thanthe second doped region and a second low conductive region beingcontiguous to an opposite side of the first doped region in the rib andexhibiting lower electric conductivity than the first doped region. Atleast one of the first and second low conductive regions may be anintrinsic region.

In a case that the rib waveguide includes the first low conductiveregion, a third doped region being contiguous to the second doped regionand the first low conductive region and having the second electricconductivity may be disposed in a region immediately below the first lowconductive region on the slab, a fourth doped region having the secondelectric conductivity may be disposed contiguous to the third dopedregion in a part of the slab in which the rib is not present on theslab, a carrier density in the first low conductive region may be lowerthan a carrier density in the third doped region, a carrier density inthe third doped region may be lower than a carrier density in the seconddoped region, and a carrier density in the fourth doped region may beequal to or higher than the carrier density in the second doped region.In a case that the rib waveguide includes the second low conductiveregion, a seventh doped region being contiguous to the first dopedregion and the first low conductive region and having the first electricconductivity may be disposed in a region immediately below the secondlow conductive region of the slab, an eighth doped region having thefirst electric conductivity may be disposed contiguous to the seventhdoped region in a part of the slab in which the rib is not present onthe slab, a carrier density in the first low conductive region may belower than a carrier density in the third doped region, a carrierdensity in the seventh doped region may be lower than the carrierdensity in the first doped region, and a carrier density in the eighthdoped region may be equal to or higher than the carrier density in thefirst doped region.

In a case that the rib waveguide includes the first low conductiveregion, a width of the second doped region may be substantially constantin a propagating direction of the guided light. In a case that the ribwaveguide includes the second low conductive region, a width of thefirst doped region may be substantially constant in the propagatingdirection of the guided light.

In a case that the rib waveguide does not include the second lowconductive region, the first doped region may be extended up to a partof the slab in which the rib is not present on the slab in the same sidewith the first doped region with respect to the boundary. In a case thatthe rib waveguide does not include the first low conductive region, thesecond doped region may be extended up to a part of the slab in whichthe rib is not present on the slab in the same side with the seconddoped region with respect to the boundary.

The optical waveguide device may further include a first metal electrodedisposed on the upper cladding. Also, a fifth doped region having thesecond electric conductivity may be disposed in the part of the slab inwhich the rib is not present on the slab in the same side with thesecond doped region with respect to the boundary, and the fifth dopedregion and the first metal electrode may be connected to each otherthrough a first through-hole via.

The optical waveguide device may further include a second metalelectrode disposed on the upper cladding. Also, a sixth doped regionhaving the first electric conductivity may be disposed in the part ofthe slab in which the rib is not present on the slab in the same sidewith the first doped region with respect to the boundary, and the sixthdoped region and the second metal electrode may be connected to eachother through a second through-hole via.

The optical waveguide device may include a first rib waveguide and asecond rib waveguide both of which constitute the two rib waveguidesdisposed parallel along a width direction of the optical waveguidedevice.

A part of the slab in the first rib waveguide closer to the second ribwaveguide than the rib in the first rib waveguide may be connected to athird metal electrode disposed on the upper cladding through a thirdthrough-hole via, and a part of the slab in the second rib waveguidecloser to the first rib waveguide than the rib in the second ribwaveguide may be connected to a fourth metal electrode disposed on theupper cladding through a fourth through-hole via.

A part of the slab in the first rib waveguide closer to the second ribwaveguide than the rib in the first rib waveguide and a part of the slabin the second rib waveguide closer to the first rib waveguide than therib in the second rib waveguide may be connected electrically to acommon fifth metal electrode disposed on the upper cladding through thethird through-hole via and the fourth through-hole via respectively.

A manufacturing method for the optical waveguide device according to asecond aspect of the present invention includes a resist production stepof, in a case that the rib waveguide includes the first low conductiveregion but does not include the second low conductive region, forming afirst resist having a resist side wall disposed in a corrugated shape inthe propagating direction of the guided light in the rib waveguide on ahorizontal surface in a location serving as a boundary between the firstlow conductive region and the second doped region, covering a regionserving as the second doped region, and exposing a region serving as thefirst low conductive region, in a case in which the rib waveguideincludes the second low conductive region but does not include the firstlow conductive region, forming a second resist having a resist side walldisposed in a corrugated shape in the propagating direction of theguided light in the rib waveguide on a horizontal surface in a locationserving as a boundary between the second low conductive region and thefirst doped region, covering a region serving as the first doped region,and exposing a region serving as the second low conductive region, and,in a case in which the rib waveguide includes the first low conductiveregion and the second low conductive region, producing the first resistor the second resist; and a resist trimming step of trimming the firstresist or the second resist after the resist production step, therebyforming a resist having a resist side wall disposed in a corrugatedshape in the propagating direction of the guided light in the ribwaveguide on a horizontal surface in a location serving as the PNjunction on a plan view of the substrate.

According to the optical waveguide device according to theabove-described aspect, it is possible to realize an optical waveguidedevice that is suitable for a small-footprint optical integrated circuitsuch as an optical modulator enabling high-speed refractive indexmodulation with low optical loss and low driving voltage and allows onlysmall quality variation for applications to optical fiber communicationdevices, in particular, optical transport equipment used for long-haulor metro-area wavelength-division multiplexing optical-fibercommunication and optical-interconnect equipment used in data centersand the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view illustrating an example of a rib on a ribwaveguide.

FIG. 2 is a schematic cross-sectional view of an optical waveguidedevice including the rib waveguide in a vertical surface including analternate long and short dash line AA′ in FIG. 1.

FIG. 3 is a schematic diagram of distribution of fringe electric fieldsin a cross-section of FIG. 2.

FIG. 4 is a schematic cross-sectional view of the optical waveguidedevice including the rib waveguide in a vertical surface including analternate long and short dash line BB′ in FIG. 1.

FIG. 5 is a schematic cross-sectional view of the optical waveguidedevice including the rib waveguide in a vertical surface includingdashed line CC′ in FIG. 1.

FIG. 6 is a graph of the reflection spectrum of the rib waveguide havingconfigurations of FIGS. 1 to 5.

FIG. 7 is a graph of the wavelength dependencies of phase shift in acase in which the refractive index profile is periodic and a case inwhich the refractive index profile is uniform.

FIG. 8 is a cross-sectional view describing the formation of the ribwaveguide.

FIG. 9 is a cross-sectional view describing the formation of a P region.

FIG. 10 is a cross-sectional view describing the formation of a P+region and a P− region.

FIG. 11 is a cross-sectional view describing the formation of an Nregion, a low conductive region (an intrinsic region), an N− region, anN+ region and a P region.

FIG. 12 is a cross-sectional view describing the formation of anintrinsic region, a P− region, a P region and a P region.

FIG. 13 is a schematic cross-sectional view of an optical waveguidedevice (including electrodes and the like) of Example 1.

FIG. 14 is a top view illustrating an example of a rib on a ribwaveguide of a second example.

FIG. 15 is a schematic cross-sectional view of an optical waveguidedevice including the rib waveguide in a vertical surface including analternate long and short dash line A₂A₂′ in FIG. 14.

FIG. 16 is a schematic cross-sectional view of the optical waveguidedevice including the rib waveguide in a vertical surface including analternate long and short dash line B₂B₂′ in FIG. 14.

FIG. 17 is a schematic cross-sectional view of the optical waveguidedevice including the rib waveguide in a vertical surface including analternate long and short dash line C₂C₂′ in FIG. 14.

FIG. 18 is a schematic cross-sectional view of an optical waveguidedevice (including electrodes and the like) of Example 2.

FIG. 19 is a block diagram of a configuration of an MZ opticalmodulator.

FIG. 20 is a schematic cross-sectional view of phase shifters of Example3 in which the optical waveguide device of Example 1 is used.

FIG. 21 is a schematic cross-sectional view of phase shifters of Example3 in which the optical waveguide device of Example 2 is used.

FIG. 22 is a schematic cross-sectional view of phase shifters of Example4 in which the optical waveguide device of Example 1 is used.

FIG. 23 is a schematic cross-sectional view of phase shifters of Example4 in which the optical waveguide device of Example 2 is used.

FIG. 24 is a schematic cross-sectional view of phase shifters of Example5 in which the optical waveguide device of Example 1 is used.

FIG. 25 is a schematic cross-sectional view of phase shifters of Example5 in which the optical waveguide device of Example 2 is used.

FIG. 26 is a schematic perspective view illustrating optical resistbefore trimming.

FIG. 27 is a schematic perspective view illustrating optical resistafter trimming.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The configuration and principle of an optical waveguide devicepreferable for high-speed refractive index modulation with low opticalloss and low driving voltage will be described. FIG. 1 schematicallyillustrates a top view of a rib on a rib waveguide (core) thatconstitutes a part of the optical waveguide device. A cross-sectionalview of the optical waveguide device including the rib waveguide isschematically illustrated in FIG. 2.

As illustrated in FIG. 2, a rib waveguide 100 is formed on a flat planarsubstrate 111. In the present embodiment, a direction in parallel to thesurface of the substrate is defined as a horizontal direction, a planarsurface in parallel to the surface of the substrate is defined as ahorizontal surface, a direction orthogonal to the surface of thesubstrate is defined as a vertical direction, and a planar surfaceorthogonal to the surface of the substrate is defined as a verticalsurface. Additionally, a direction in parallel to the horizontaldirection and orthogonal to the propagating direction of guided light isdefined as a width direction.

The top view in FIG. 1 illustrates the configuration of a rib 114 in therib waveguide 100 on the horizontal surface. For elements constitutingthe optical waveguide device, widths are measured in the horizontaldirection, and heights are measured in the vertical directionrespectively. The rib waveguide 100 is a waveguide core including therib 114 and a slab 115, and is formed of crystalline silicon (Si). Therib 114 is a single protrusion (also called “a ridge”) which extends inthe propagating direction of guided light, and the rib 114 is disposedon the center part of the slab 115 in width direction. The rib waveguidecan be formed by removing both sides of the rib in the width directionfrom the flat and uniform core by etching, etc. In case that the rib isformed of polycrystalline silicon, rib waveguide can be formed bydepositing the polycrystalline silicon on the crystalline silicon havingthe thickness of the slab, and removing both sides of thepolycrystalline silicon in the width direction by photolithography andetching.

Claddings around the rib waveguide 100, that is, a lower cladding 112and an upper cladding 113 are formed of silicon dioxide, that is, silica(SiO₂). The width w_(rib) of the rib 114 in the rib waveguide 100 is,for example, in a range of 500 nm to 600 nm. The propagating directionof guided light in the rib waveguide is the horizontal direction, and isa direction orthogonal to the width w_(rib). The width w_(rib) isconstant in the propagating direction of the guided light.

As illustrated in FIG. 1, P region 101 doped with P-type dopants and Nregion 102 doped with N-type dopants are disposed in a corrugateddistribution pattern in the propagating direction of guided light. Inthe embodiment, for example, boron and phosphorous are used as P-typedopant and N-type dopant respectively, but the dopants are not limitedthereto, and other elements that can serve as P-type dopants (nitrogen,arsenic, antimony and the like) and other elements that can serve asN-type dopants (aluminum, indium and the like) may be used.

A half period of the corrugated pattern in the propagating direction ofguided light is represented by d, and thus a period is represented by2d. The width w_(P) of the P region 101 and the width w_(N) of the Nregion 102 are substantially constant in the propagating direction ofthe guided light respectively. A PN junction is formed at a boundary 103between the P region 101 and the N region 102. As described below, the Pregion 101 and the N region 102 are present across a region of the rib114 and the slab 115 immediately below the rib 114 in the rib waveguide100. In the vertical direction, the P region 101 and the N region 102are present from a top surface 116 of the rib 114 to a bottom surface117 of the slab 115. In the horizontal direction, the total width of theP region 101 and the N region 102 is narrower than the width w_(rib) ofthe rib 114. In addition, the P region 101 and the N region 102 extendin the propagating direction of guided light within a limited range inaccordance with purposes such as refractive index modulation. Theboundary 103 is formed from the top surface 116 of the rib 114 to thebottom surface 117 of the slab 115 in the vertical direction, and formsa corrugated line extending in the propagating direction of the guidedlight in the horizontal direction.

Reverse bias voltage is applied to the optical waveguide device of FIG.1, and a depletion region 104 is present along the boundary 103 andextends toward the P region 101 and the N region 102 from the PNjunction. The width of the depletion region 104 extending toward the Pregion 101 and the N region 102 increases in proportion to the absolutevalue of the reverse bias voltage. The range within which the positionof the boundary 103 changes in a direction along w_(rib) is denoted bythe width w_(c). Examples of the values of the widths are as follows: ina case in which w_(rib) is 500 nm, w_(p) is 120 nm, w_(N) is 90 nm, andw_(c) is 130 nm.

An intrinsic region 105 is a low conductive region having a lowerelectric conductivity than the P region 101. Also, the intrinsic region106 is a low conductive region having a lower electric conductivity thanthe N region 102. It becomes possible to reduce optical loss andparasitic capacitance using an intrinsic region 105 having a reducedconcentration of the P-type dopant (having a lower electric conductivitythan the P region 101) and an intrinsic region 106 having a reducedconcentration of the N-type dopant (having a lower electric conductivitythan the N region 102). The intrinsic region 105 and the intrinsicregion 106 extend in the horizontal direction along the propagatingdirection in a corrugation line. The period of the corrugation is equalto the periods of the P region 101 and the N region 102. The maximumwidth of the intrinsic region 105 is represented as w_(IP), and themaximum width of the intrinsic region 106 is represented as w_(IN). Forexample, both of w_(IP) and w_(IN) are 210 nm. In addition, in a case inwhich w_(rib) is 600 nm, w_(C) is 230 nm. However, the widths of therespective sections are not limited to the above-described example, andmay be optimized with respect to of optical loss, driving voltage andoperation speed as well as the dimensional accuracy of fabricationprocesses.

Due to limitations of fabrication accuracy, a maximum of approximately±80 nm of fabrication error is produced in w_(P), w_(N), w_(IP) andw_(IN). To suppress performance variation and enhance mass productivity,it is preferable to optimize the fabrication processes and thus improvethe fabrication accuracy, thereby suppressing the fabrication error toapproximately ±40 nm or less. In an example of the invention, w_(P) andw_(N) are substantially constant in the propagating direction of theguided light. However, w_(P) and w_(N) are not always required to beconstant, and w_(P) and w_(N) may be changed to make w_(IP) and w_(IN)substantially constant in the propagating direction of the guided light.In the corrugation of the position of the boundary of the PN junction,the above-described fabrication error is present at the positions ofcrests and troughs. At the positions of crests and troughs as well, thefabrication error is preferably approximately ±40 nm or less.

FIG. 2 schematically illustrates a schematic cross-sectional view of theoptical waveguide device including the rib waveguide in the verticalsurface including an alternate long and short dash line AA′ in FIG. 1.The alternate long and short dash line AA′ passes through one of thosepoints at which the boundary 103 is positioned on the center of w_(rib)on a straight axis along the propagating direction of the guided light,and is drawn in parallel to w_(rib). FIG. 2 illustrates a part ofconstituents inside of the optical waveguide device including the ribwaveguide 100, the substrate 111, the lower cladding 112 and the uppercladding 113, but does not illustrate metal electrodes and doped regionsin contact with the metal electrodes. The lower cladding 112 is disposedon the substrate 111, the rib waveguide 100 is disposed on the lowercladding 112, and the upper cladding 113 is disposed on the ribwaveguide 100. Crystalline Si is used for the substrate 111, and SiO₂ isused for the lower cladding 112 and the upper cladding 113. The lowercladding 112 and the upper cladding 113 are substantially planar.

The PN junction is formed on the boundary 103 between the P region 101and the N region 102, and, under reverse bias voltage, the depletionregion 104 is formed along the boundary 103 and extends toward the Pregion 101 and the N region 102 of the boundary 103. The rib 114includes side walls 118 and 119 which are respectively disposed on bothsides of the rib in the width direction. The intrinsic region 105 isdisposed between a side wall 118 that is a side wall of the rib 114closer to the P region 101 than to the N region 102 and the P region101. The intrinsic region 105 is contiguous to the side wall 118 of therib 114 in the rib waveguide 100. The intrinsic region 106 is disposedbetween a side wall 119 that is a side wall of the rib 114 closer to theN region 102 than to the P region 101 and the N region 102. Theintrinsic region 106 is contiguous to the side wall 119 of the rib 114in the rib waveguide 100.

As illustrated in FIG. 1, the intrinsic region 105 is contiguous to theP region 101, extends in a direction opposite to the PN junction awayfrom the P region 101 in the rib 114, and is contiguous to the side wall118. Similarly, the intrinsic region 106 is contiguous to the N region102, extends in a direction opposite to the PN junction away from the Nregion 102 in the rib 114, and is contiguous to the side wall 119. Thatis, there are no doped regions between the intrinsic region 105 and theside wall 118, and between the intrinsic region 106 and the side wall119. The carrier density can be changed discontinuously or continuously,in a single step or multiple steps between the intrinsic region 105 andthe P region 101, and between the intrinsic region 106 and the N region102.

A P− region 107 is disposed in the slab 115 below the intrinsic region105. The P− region 107 is disposed in a region including the slab 115immediately below the intrinsic region 105, and is contiguous to the Pregion 101 and the intrinsic region 105. The carrier density in theintrinsic region 105 is lower than the carrier density in the P− region107. The electric conductivity of the P− region 107 is P-type, that isthe same as the P region 101, and the carrier density in the P− region107 is lower than the carrier density in the P region 101. A P region108 is disposed in the slab 115 outside the P− region 107 (the oppositeside to the P region 101). The P region 108 is contiguous to the P−region 107, but is not to the P region 101. The electric conductivity ofthe P region 108 is P-type, that is the same as the P region 101. Thecarrier density in the P region 108 is equal to the carrier density inthe P region 101. The P region 108 is disposed in a part of the slab 115in which the rib 114 is not present on the slab 115, and is contiguousto a top surface 120 of the slab 115.

On the other hand, an N− region 109 is disposed in the slab 115 belowthe intrinsic region 106. The N− region 109 is disposed in a regionincluding the slab 115 immediately below the intrinsic region 106, andis contiguous to the N region 102 and the intrinsic region 106. Thecarrier density in the intrinsic region 106 is lower than the carrierdensity in the N− region 109. The electric conductivity of the N− region109 is N-type, that is the same as the N region 102, and the carrierdensity in the N− region 109 is lower than the carrier density in the Nregion 102. An N+ region 110 is disposed in the slab 115 outside the N−region 109 (the opposite side to the N region 102). The N+ region 110 iscontiguous to the N− region 109, but is not to the N region 102. Theelectric conductivity of the N+ region 110 is N-type, that is the sameas the N region 102, and the carrier density in the N+ region 110 ishigher than the carrier density in the N region 102. The N+ region 110is disposed in a part of the slab 115 in which the rib 114 is notpresent on the slab 115, and is contiguous to the top surface 120 of theslab 115.

Similarly to the N+ region 110 disposed outside the N− region 109, it isalso possible to dispose a P+ region having a higher carrier densitythan the carrier density in the P region 101 outside the P− region 107instead of the P region 108. In addition, similarly to the P region 108disposed outside the P− region 107, it is also possible to provide an Nregion having the same carrier density as the carrier density in the Nregion 102 outside the N− region 109 instead of the N+ region 110.

In the embodiment, one electric conductivity selected from the electricconductivity of the P-type dopant (p-type electric conductivity) and theelectric conductivity of the N-type dopant (n-type electricconductivity) is defined as a first electric conductivity, and the otherelectric conductivity is defined as a second electric conductivity, bothof which are different each other. In certain embodiments of theinvention, in a single rib waveguide (core), a region having the firstelectric conductivity is disposed on the left side of the center of therib in the width direction in the rib waveguide (core) in the horizontaldirection, and a region having the second electric conductivity isdisposed on the right side of the center of the rib in the widthdirection in the rib waveguide (core) in the horizontal directionrespectively. In other embodiments, while the disposition is reversed inthe horizontal direction (the width direction), the optical waveguidedevice including a single rib waveguide (core) obtains the same effect.In addition, even when the definitions of the first electricconductivity and the second electric conductivity are switched eachother, the same effect can be obtained. That is, it does not make anydifference whether the P-type electric conductivity is defined as thefirst conductivity and the N-type electric conductivity is defined asthe second conductivity, or the N-type electric conductivity is definedas the first conductivity and the P-type electric conductivity isdefined as the second conductivity.

A P++ region and an N++ region which have an extremely high carrierdensity and are suitable for electric connection with the metalelectrodes are provided at outside parts away from the rib 114 in thehorizontal direction (the width direction) in regions of the slab 115 onwhich the rib 114 is not present (refer to a P++ region 251 and an N++region 252 in FIG. 13). Bias voltage is applied in the horizontaldirection (the width direction) between the P++ region and the N++region.

In the optical waveguide device having the boundary 103 along thecorrugated PN junction schematically illustrated in FIG. 1, it ispossible to extend the effective length of the PN junction in thepropagating direction of the guided light compared with an opticalwaveguide device having a boundary along a straight PN junction, andthereby to reduce the driving voltage. Optical-mask alignment error inphotolithography process to form the boundary 103 is averaged andthereby removed with the distribution of the corrugated PN junction.Therefore, it is possible to reduce performance variations in individualchips. When the effective length of the PN junction is extended, opticalloss increases and parasitic capacitance increases, thereby theresistance-capacitance (RC) time constant increases and the operationspeed decreases. Thus, it is necessary to introduce a mechanismeliminating the above-described disadvantages. According to theembodiment, it becomes possible to avoid both an increase in opticalloss and an increase in parasitic capacitance as described below.

In regions adjacent to the side walls 118 and 119 on both sides of therib 114 in the rib waveguide 100, the carrier density does not changesubstantially even when the bias voltage is changed. When these adjacentregions are formed as the intrinsic region 105 and the intrinsic region106, it is possible to reduce the optical loss of the rib waveguide. Inthe vicinities of the sections immediately below the intrinsic region105 and the intrinsic region 106 in the slab below the core, the carrierdensity only slightly changes as the bias voltage changes. When thevicinities are formed as the P− region 107 and the N− region 109 whichhave a low carrier density, it is possible to reduce the optical loss ofthe rib waveguide. When the bias voltage is applied, since the P− region107 and the N− region 109 cause series resistances respectively, it isnecessary to optimize the respective resistances to avoid impairment ofhigh-speed operation.

Coulomb interaction becomes weak, and thereby, the fringe capacitancebetween the intrinsic region 105 and the intrinsic region 106 and thefringe capacitance between the P− region 107 and the N− region 109become negligible, when the carrier density is reduced. FIG. 3schematically illustrates the distribution of fringe electric fields inthe cross-section of FIG. 2. Curves indicate the lines of electric forcecorresponding to the fringe electric fields. To illustrate the lines ofelectric force clearer, the reference signs to constituents are notindicated. In regions having a dense distribution of the lines ofelectric force, Coulomb interaction is strong, and capacitance is high.The fringe capacitances in the P region 101 and the N region 102 becomedominant. The fringe capacitance becomes parasitic capacitance, andimpairs high-speed operation. When the intrinsic region 105, theintrinsic region 106, the P− region 107 and the N− region 109 areintroduced into the rib waveguide (core), it is possible to reduce theparasitic capacitance, and the high-speed refractive index modulation ofthe optical waveguide device of the embodiment becomes possible.

A schematic cross-sectional view of the rib waveguide in a verticalsurface including an alternate long and short dash line BB′ in FIG. 1 isillustrated in FIG. 4. In FIG. 4, the reference signs to the respectiveconstituents are the same as in FIG. 2. The alternate long and shortdash line BB′ passes through a point at which the P region 101 isclosest to the side wall 118 of the rib 114 on the opposite side of theboundary 103 within a range of the period 2d on the straight axis alongthe propagating direction of the guided light, and is drawn in parallelwith w_(rib). At this point, the width of the intrinsic region 106becomes largest in a direction along the alternate long and short dashline BB′. The width is represented by w_(IN).

A schematic cross-sectional view of the rib waveguide in a verticalsurface including an alternate long and short dash line CC′ in FIG. 1 isillustrated in FIG. 5. In FIG. 5, the reference signs to the respectivecomponents are the same as in FIG. 2. The alternate long and short dashline CC′ passes through a point at which the N region 102 is closest tothe side wall 119 of the rib 114 on the opposite side of the boundary103 within a range of the period 2d on a straight axis along thepropagating direction of the guided light, and is drawn in parallel withw_(rib). At this point, the width of the intrinsic region 105 becomeslargest in a direction along the alternate long and short dash line CC′.The width is represented by w_(IP).

When carriers are distributed by doping P-type or N-type dopant to Si,the refractive index decreases. In the rib waveguides having theconfiguration of FIGS. 1 to 5, since the refractive index isperiodically distributed in the propagating direction of the guidedlight, it is not possible to neglect the effect of Bragg reflection.Since the periodic distribution of the dopant causes a periodic profileof a low refractive index in Si that is a base medium, Bragg gratinghaving a negative refractive index contrast is formed. When a reflectionspectrum is obtained using a transfer matrix method (TMM) assuming asquare-waveform refractive index profile having abrupt interfaces in thepropagating direction of the guided light, the reflection spectrum isexpressed as illustrated in a graph in FIG. 6. Here, the Braggwavelength is 1570.1 nm, and the length of the rib waveguide in thepropagating direction of the guided light is 3 mm.

When the intrinsic region 105, the intrinsic region 106, the P− region107 and the N− region 109 are introduced into the rib waveguide (core),the contrast of the periodic change of the refractive index in the ribwaveguide becomes strong, and thereby, strong reflection is causedaround the Bragg wavelength, and a stopband is generated as shown inFIG. 6. To obtain a return loss of 30 dB or more by suppressingreflection, it is necessary to separate the wavelength range to be usedand the Bragg wavelength by 5 nm or more. In the corrugation profile ofFIG. 1, while the period is 2d in the physical structure, the period ofthe refractive index profile projected on the axis along the propagatingdirection of the guided light, that is, the period d_(G) of the Bragggrating becomes one of d and 2d, since w_(IP)=w_(IN), and the effectiverefractive index in the vertical surface including the alternate longand short dash line BB′ and the effective refractive index in thevertical surface including the alternate long and short dash line CC′are substantially the same. In the above-described derivation of thereflection spectrum using TMM, d_(G) is set to d. Since the stopbandcorresponding to d_(G)=2d is generated at a wavelength as twice as thewavelength of the stopband corresponding to d_(G)=d, the stopband issubstantially negligible in a wavelength range in the vicinity of astopband corresponding to d_(G)=d. However, depending on conditions,there are cases in which, a phenomenon, whereby influence of a higherorder stopband is enhanced, occurs, and the period d_(G) of the Bragggrating becomes equal to the period 2d in the physical structure, and ddoes not equal to the period d_(G) of the Bragg grating, even when theintrinsic regions are disposed on the both sides of the rib in the widthdirection.

The optical waveguide device of the present invention can be preferablyused in C and L bands that are optical communication wavelength bands.In this case, d_(G) is adjusted so as to make the C and L bandscorrespond to a wavelength region in a short wavelength side of thestopband. The reason for using the short wavelength side of the stopbandwill be described below. When w_(rib) is set to 500 nm, the heighth_(rib) from the bottom surface 117 of the slab 115 to the top surface116 of the rib 114 is set to 220 nm, the height h_(slab) from the bottomsurface 117 of the slab 115 to the top surface 120 of the slab 115 isset to 95 nm, and the wavelength is set to 1550 nm, the effectiverefractive index n_(eff) of the optical waveguide including the ribwaveguide (Si core) and the SiO₂ claddings of the invention is 2.6. Thechange in n_(eff) caused by a variation in w_(rib) due to fabricationerror is smaller than 1%. When a margin of 5 nm is added to thewavelength (1620 nm) of the long-wavelength end of the L band so as toobtain a wavelength of 1625 nm as the Bragg wavelength λ_(G) to ensure areturn loss of 30 dB or more, d_(G) is 313 nm based on Formula (1).

$\begin{matrix}{d_{G} = \frac{\lambda_{G}}{2n_{eff}}} & (1)\end{matrix}$

Since d_(G)=d, when d≧313 nm, it is possible to obtain a return loss of30 dB or more in the C and L bands. In addition, when w_(rib) is set to600 nm, since n_(eff) becomes larger than 2.6, λ_(G) is further shiftedtoward longer wavelength side with the same d, and the return lossfurther increases. As a result, it is possible to suppress the Braggreflection. When high-accuracy fabrication processes are employed,tolerance margin for the variation is not required, and thelong-wavelength end of a desired wavelength band may be set to theabove-described Bragg wavelength.

When the reflective index is periodically distributed in the propagatingdirection of the guided light, it is possible to reduce the wavelengthdependence of phase shift as illustrated in FIG. 7. This is preferablewhen applied to wavelength-division multiplexing transmission. Theoptical characteristics in a case in which the refractive index isperiodically distributed as illustrated in FIG. 7 (periodic) werederived using the same model as for the reflection spectrum of FIG. 6.For comparison, the characteristics of a case in which the boundary ofthe PN junction does not change in a corrugated line but remains in auniform straight line (uniform) are plotted in FIG. 7. The wavelengthdependence of the phase shift in the wavelength bands on both sides ofthe stopband is reduced. In the case of the uniform profile, thewaveguide length necessary to generate a certain phase shift isproportional to the wavelength of the guided light, and therefore thephase shift decreases as the wavelength increases. In the case of theperiodic profile, phase enhancement induced by the multiple reflectionoccurs, and the wavelength dependence of the phase shift is reduced.This is an advantage of the use of Bragg grating having a negative indexcontrast.

When the short wavelength side of the stopband is used for transmission,since the stopband corresponding to the period 2d is further separatedfrom the wavelength band to be used, the use of the short wavelengthside is effective in order to increase the return loss by reducing theBragg reflection. Furthermore, when the short wavelength side is used,the Bragg wavelength becomes longer than the C and L bands. Therefore, dbecomes larger with respect to the fabrication errors and thefabrication accuracy is improved, and thereby, it becomes easier toreduce quality variation.

Example 1

The fabrication method and configuration of an optical waveguide devicethat functions based on the above-described configuration and principlewill be described in detail using the cross-sectional configuration ofFIG. 2 as an example. A rib waveguide 200 illustrated in FIG. 8 isfabricated on a silicon-on-insulator (SOI) layer using an SOI waferthrough photolithography and etching. The substrate in an SOI waferserves as a substrate 211, and a buried oxidized layer in the SOI waferserves as a lower cladding 212. The thickness of the lower cladding 212in the vertical direction is 2 μm.

An optical resist is applied onto the rib waveguide 200, and an opticalresist 221 having a cross-sectional shape illustrated in FIG. 9 isobtained through photolithography. A top surface 216 and side walls 218and 219 on both sides of a rib 214 in the center of the rib waveguide200, the entire area of a top surface 220A of a slab 215 on the leftside of the rib 214 in the width direction (left side of the rib 214),and an area in a top surface 220B located on the side opposite to thetop surface 220A in the width direction are exposed by removing theoptical resist, with the area of the top surface 220A being contiguousto the rib 214. The entire top surface 220A and the part of the topsurface 220B of the slab 215 are exposed parts on which the rib 214 isnot present.

The horizontal distance w₁ from the center of w_(rib), that is, thecenter of the rib 214 in the width direction on the horizontal surfaceto a side wall 222 of the optical resist 221 is 700 nm. Boron that isP-type dopant is implanted into regions in which the surface of the ribwaveguide 200 is exposed, that is, the exposed parts through the ionimplantation, and a P region 223 is formed. A part immediately below theoptical resist 221 forms an undoped region 224.

After removing the optical resist 221, an optical resist is applied, andan optical resist 231 having a cross-sectional shape of FIG. 10 isformed. A side wall 232 of the optical resist 231 has a corrugated shapeas illustrated in FIG. 26. The optical resist 231 covers the entire areaof the top surface 220A of the slab 215 which is located on the leftside of the rib 214 and a part of the top surface 216 of the rib 214.Boron is ion-implanted into the inside from the regions in which thesurface of the rib waveguide 200 is exposed, and a P+ region 233 isobtained. The ion implantation is carried out in two steps. In the P+region 233, based on the surface of the slab section as a boundary, thecarrier density in the region above the boundary is higher than thecarrier density in the region below the boundary. Due to ion diffusion,the carrier density changes smoothly (graded) rather than abruptly inthe boundary. Ion acceleration voltage is adjusted in individual stepsto satisfy relation ship of the carrier density described below. Theundoped region 224 in FIG. 9 is converted to a P− region 234 in FIG. 10.The electric characteristics of the P region 223 immediately below theoptical resist 231 do not change.

A cross-sectional shape illustrated in FIG. 11 is obtained by carryingout trimming (refer to FIG. 27) without removing the optical resist 231.The position of a side wall 235 of the optical resist 231 is moved backon the top surface 216 of the rib 214, and a wider range of the topsurface 216 in the width direction is exposed compared with the rangebefore trimming. In FIGS. 26 and 27, the doped regions are notdistinguished and similar hatching is carried out on the cross-sectionof the rib waveguide 200. The shape of the side wall 232 before trimmingin FIGS. 10 and 26 corresponds to a corrugated boundary between an Nregion 236 and an intrinsic region 237 in FIGS. 11 to 13. The shape ofthe side wall 235 after trimming in FIGS. 11 and 27 corresponds to acorrugated boundary of a PN junction in FIGS. 12 and 13. Since the twocorrugated boundaries can be formed using one optical resist 231, it ispossible to suppress the position deviation of the corrugated patternscompared with a case of corrugated pattern formation with removal of theoptical resist 231 and subsequent patterning of a new optical resist.Examples of a trimming process include a process step in which a opticalresist is partially oxidized and removed using O₂ plasma etching or thelike. The moved-back distance (displacement) of the resist side wall canbe adjusted depending on the intensity, time and the like of theO₂-plasma treatment. In a case in which the thickness of the opticalresist decreases during trimming, it is necessary to procure asufficiently thick optical resist in advance.

Phosphorous that is N-type dopant is ion-implanted into the regions inwhich the surface of the rib waveguide 200 is exposed, and a section inwhich the surface of the P region 223 is exposed is converted to the Nregion 236. The P+ region 233 in FIG. 10 is converted to the intrinsicregion 237 and an N− region 238. The P− region 234 in FIG. 10 isconverted to an N+ region 239. The intrinsic region 237 is contiguous toa side wall on the right side of the rib 214. The N− region 238 isformed in the slab section immediately below the intrinsic region 237.The N region 236, the intrinsic region 237 and the N− region 238 arecontiguous to each other at their boundaries. The N− region 238 and theN+ region 239 are contiguous to each other through at their boundary.The electric characteristics of the P region 223 immediately below theoptical resist 231 do not change. A PN junction is formed along aboundary 240 between the P region 223 and the N region 236. Thecross-sectional view of FIG. 11 illustrates a PN junction under zerobias voltage. The width of the depletion region is negligible and isthus not shown.

After removing the optical resist 231, optical resists 241 and 242having cross-sectional shapes illustrated in FIG. 12 are obtainedthrough application of an optical resist and subsequent photolithographyprocess. The horizontal distance w₂ from the center of the rib 214 inthe width direction on the horizontal surface to a side wall 243 of theoptical resist 231 is 700 nm. A side wall 244 of the optical resist 242has a corrugated shape. Phosphorous that is N-type dopant ision-implanted into the inside from the regions in which the surface ofthe rib waveguide 200 is exposed in two steps, and a region immediatelybelow the section in which the surface of the P region 223 is exposed isconverted to an intrinsic region 245 and a P− region 246. The intrinsicregion 245 is contiguous to a side wall on the left side of the rib 214.The P-region 246 is formed in the slab 215 immediately below theintrinsic region 245. The electric characteristics of individualsections of the P region 223 immediately below the optical resists 241and 242 do not change; however, for the sake of discrimination, asection immediately below the optical resist 241 is represented as a Pregion 247, and a section immediately below the optical resist 242 isrepresented as a P region 248. The P region 248, the intrinsic region245 and the P− region 246 are contiguous to each other at theirboundaries. The P− region 246 and the P region 247 are contiguous toeach other at their boundary.

After removing the optical resists 241 and 242, to reduce contactresistance in electric connection, the P++ region 251 is formed in apart of the P region 247 through ion implantation as illustrated in FIG.13. Furthermore, the N++ region 252 is formed in a part of the N+ region239 through ion implantation. The electric characteristics of theremaining sections of the P region 247 and the N+ region 239 do notchange. The order of the formation process steps of the P++ region 251and the N++ region 252 may be reversed.

When the carrier densities in the intrinsic region 245, the P− region246, the P region 248 and the P++ region 251 are denoted by p_(IP),p_(P−), p_(P) and p_(P++) respectively, and the carrier densities in theintrinsic region 237 the N− region 238, the N region 236, the N+ region239 and the N++ region 252 by n_(IN), n_(N−), n_(N), n_(N+) and n_(N++)respectively, the following relation formulae are satisfied.p _(IP) <p _(P−) <p _(P) <p _(P++)  (2)n _(IN) <n _(N−) <n _(N) <n _(N+) <n _(N++)  (3)

For example, p_(IP) and n_(IN) are smaller than 1×10¹⁷ cm⁻³, and arepreferably 1×10¹⁶ cm⁻³ or less. p_(P) and n_(N) are, for example, in arange of 1×10¹⁸ cm⁻³ to 5×10¹⁸ cm⁻³, and p_(P++) and n_(N++) are 1×10¹⁹cm⁻³ or more. The carrier density in the P region 247 is equal to p_(P).In the following examples, the same relation formulae are satisfied.However, the values of the carrier density are not necessarily limitedto the exemplified value ranges, and may be set to be appropriate tosuit low driving voltage, low optical loss, and high-speed operation.

An upper cladding 213 having a thickness of 2 μm in the verticaldirection is formed by depositing SiO₂ on the rib waveguide 200.Through-hole vias are formed in the upper cladding 213, and viaelectrodes 253 and 254 for vertical interconnect access are formed.Traveling-wave electrodes 255 and 256 are formed on the upper cladding213. The traveling-wave electrodes 255 and 256 are a part of atraveling-wave electrode propagating high-frequency electric signalssuch as a coplanar waveguide or a coplanar stripline. The via electrodes253 and 254 and the traveling-wave electrodes 255 and 256 are formed ofaluminum. When a high-frequency electric signal is applied to thetraveling-wave electrode 255 or 256, the width of the depletion layeralong the PN junction in the boundary 240 under reverse bias or anelectric current flowing along the PN junction in the boundary 240 underbias is modulated, and the refractive index of the rib waveguide ismodulated at a high speed.

Example 2

In the present example, a second configuration of the optical waveguidedevice of the invention will be described. FIG. 14 schematicallyillustrates a top view illustrating a rib in the example. Across-sectional view of an optical waveguide device including the ribwaveguide is schematically illustrated in FIG. 15. Since theconstituents of a substrate 311, a lower cladding 312, an upper cladding313, a rib 314 and a slab 315 in a rib waveguide 300 are same as thoseof the substrate 111, the lower cladding 112, the upper cladding 113,the rib 114 and the slab 115 in the rib waveguide 100 in Example 1respectively, they will not be described again here.

A P-doped P region 301 and an N-doped N region 302 are distributed incorrugated shapes in the propagating direction of the guided light. Thewidth w_(N2) of the N region 302 is substantially constant in thepropagating direction of guided light, and w_(N2)=w_(N) (refer to FIG.1). For example, when w_(rib2) is 500 nm, w_(N2) is 90 nm. On the otherhand, the width of the P region 301 changes in the propagating directionof guided light in a range of w_(P2) to w_(P3).

Along a boundary 303 between the P region 301 and the N region 302, a PNjunction is formed. In the example as well, reverse bias voltage isapplied to the optical waveguide device, and a depletion region 304extends from the boundary 303 toward the P region 301 and the N region302 of the PN junction. The period of the corrugation patterns is 2d₂.The P region 301 and the N region 302 are disposed across a region ofthe rib 314 and the slab 315 immediately below the rib 314 in the ribwaveguide 300. In the vertical direction, the P region 301 and the Nregion 302 are disposed from a top surface 316 of the rib 314 to abottom surface 317 of the slab 315. In the horizontal direction, the Pregion 301 and the N region 302 extend in the propagating direction ofthe guided light within a limited range in accordance with purposes suchas refractive index modulation. Similarly to Example 1, the boundary 303is formed from the top surface 316 of the rib 314 to the bottom surface317 of the slab 315 in the vertical direction, and extends in acorrugated line in the propagating direction of the guided light in thehorizontal direction.

The width w_(rib2) of the rib 314 is equal to that in Example 1 (referto FIG. 1), and w_(rib2)=w_(rib). The range within which the position ofthe boundary 303 changes in a direction along w_(rib2) is denoted by thewidth w_(c2). w_(c2) is equal to the value of w_(c) in Example 1. Forexample, when w_(rib2) is 500 nm, w_(c2) is 130 nm. When the minimumvalue and maximum value of the width of the P region 301 are denoted byw_(P2) and w_(P3), w_(P2)=200 nm and w_(P3)=330 nm. In this case, w_(p2)is equal to the sum of w_(p2) and w_(cp).

In the rib waveguide of the example as well, the refractive index isperiodically distributed in the propagating direction of the guidedlight so that Bragg reflection is caused and Bragg grating having anegative refractive index contrast is formed. In the example, anintrinsic region is provided only on one side of the rib in the widthdirection, and, since the effective refractive index in the verticalsurface including the alternate long and short dash line B₂B₂′ and theeffective refractive index in the vertical surface including thealternate long and short dash line C₂C₂′ are different each other, theperiod d_(G) of the Bragg grating is equal to the period 2d₂ in thephysical structure. That is, d_(G)=2d₂. Therefore, when 2d₂≧313 nm, itis possible to obtain a return loss of 30 dB or more in the C and Lbands.

A part of constituents of the optical waveguide device including the ribwaveguide in a vertical surface including an alternate long and shortdash line A₂A₂′ is illustrated in a schematic cross-sectional view ofFIG. 15. The alternate long and short dash line A₂A₂′ passes through oneof those points at which the boundary 303 is located in the center ofw_(rib2) on the straight axis along the propagating direction of theguided light, and is drawn in parallel to w_(rib2). The lower cladding312, the rib waveguide 300 and the upper cladding 313 are disposed onthe substrate 311. In the rib waveguide 300, the P region 301, the Nregion 302, the boundary 303 between the P region 301 and the N region302, the depletion region 304 along the PN junction on the boundary 303,an intrinsic region 306 contiguous to a side wall 319 of the rib 314 inthe rib waveguide 300, an N− region 309 disposed in the slab 315immediately below the intrinsic region 306, and an N+ region 310 outsidethe N− region are formed. The P region 301 extends up to a part of theslab 315 in which the rib 314 is not present on the slab 315.

A schematic cross-sectional view of the optical waveguide deviceincluding the rib waveguide in a vertical surface including an alternatelong and short dash line B₂B₂′ is illustrated in FIG. 16. The alternatelong and short dash line B₂B₂′ passes through a point at which the Nregion 302 is farthest from the side wall 319 of the rib 314 on theopposite side of the boundary 303 within a range of the period 2d₂ onthe straight axis along the propagating direction of the guided light,and is drawn in parallel to w_(rib2). At this point, the width of theintrinsic region 306 becomes largest in the direction along thealternate long and short dash line B₂B₂′. The width is denoted byw_(IN2). _(wIN2)=_(wIN) (refer to FIG. 1). For example, when w_(rib2) is500 nm, w_(IN2) is 210 nm.

A schematic cross-sectional view of the optical waveguide deviceincluding the rib waveguide in a vertical surface including an alternatelong and short dash line C₂C₂′ is illustrated in FIG. 17. The alternatelong and short dash line C2C2′ passes through a point at which the Pregion 101 is closest to the side wall 319 of the rib 314 on theopposite side of the boundary 303 within a range of the period 2d on thestraight axis along the propagating direction of the guided light, andis drawn in parallel to w_(rib2).

A schematic cross-sectional view of the optical waveguide device of theexample is illustrated in FIG. 18. In the fabrication of the presentdevice, the same fabrication processes as in Example 1 are adopted up tothe fabrication step of the N region 236, the intrinsic region 237, theN− region 238, the N+ region 239 and the P region 223 in FIG. 11. Afterthat, the P++ region 251 and the N++ region 252 are formed through ionimplantation. The upper cladding 213 having a thickness of 2 μm in thevertical direction is formed by depositing SiO₂ on the rib waveguide200. Through-hole vias are formed in the upper cladding 213, and viaelectrodes 253 and 254 are formed. The traveling-wave electrodes 255 and256 are fabricated on the upper cladding 213. w_(P4) is a mean value ofw_(P2)=200 nm and w_(P3)=330 nm, and is 265 nm.

In the optical waveguide device of the example, there is no intrinsicregion adjacent to a side wall of the rib closer to the P region 223than to the N region 236 as illustrated in FIG. 18, and simplerfabrication is possible. Therefore, optical characteristic variationcaused by fabrication errors becomes smaller than that of the device ofExample 1, and thereby, it is possible to provide an optical waveguidedevice having smaller quality variation.

Example 3

A configuration of an optical waveguide device functioning as aMach-Zehnder (MZ) optical modulator will be described using the opticalwaveguide device described in Example 1 or 2. A block diagram of theconfiguration of the MZ optical modulator is illustrated in FIG. 19. TheMZ optical modulator consists of the following components:

input waveguide 1905;

1×2 splitter section 1903;

first arm consisting of a waveguide 1906, a phase shifter 1901 and awaveguide 1908; second arm consisting of a waveguide 1907, a phaseshifter 1902 and a waveguide 1909;

2×1 coupler section 1904; and

output waveguide 1910.

An input port of the 1×2 splitter section 1903 is connected to the inputwaveguide 1905, and two output ports of the 1×2 splitter section 1903are respectively connected to the two arms. Two input ports of the 2×1coupler section 1904 are respectively connected to the two arms, and anoutput port of the 2×1 coupler section 1904 are connected to the outputwaveguide 1910.

The input waveguide 1905, the waveguides 1906, 1907, 1908 and 1909, andthe output waveguide 1910 have a rectangular silicon core. The width ofthe rectangular core is equal to the width of the rib in the ribwaveguide described in Example 1 or 2. The height of the square core isequal to the height (h_(rib) in FIG. 8) of the rib waveguide. In aconnection section between the rectangular core and the rib waveguide,an upper section of the square core is connected to the rib in the ribwaveguide, and a lower section of the square core is connected to theslab in the rib waveguide.

The MZ optical modulator in FIG. 19 is fabricated through monolithicintegration on an SOI wafer. Each of the phase shifters 1901 and 1902has a single rib waveguide (core), and is formed adjacent to each otheron an SOI substrate. The MZ optical modulator in FIG. 19 is an opticalwaveguide device including a plurality of rib waveguides (cores).

FIG. 20 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 1 on a vertical surface including an alternate longand short dash line DD′. The left half of the schematic cross-sectioncorresponds to the phase shifter 1901 and the right half corresponds tothe phase shifter 1902.

A lower cladding 2038 is disposed on a substrate 2037. A rib waveguide2001 that serves as a core of a waveguide in the phase shifter 1901 anda rib waveguide 2021 that serves as a core of a waveguide in the phaseshifter 1902 are disposed on the lower cladding 2038. An upper cladding2039 is disposed on the rib waveguides 2001 and 2021. An SOI layerbetween the rib waveguide 2001 and the rib waveguide 2021 is removed,and silica is loaded. In a case in which electric conductance betweenthe phase shifter 1901 and the phase shifter 1902 through the SOI layeris negligible, it is not necessary to remove the SOI layer. Thedescription of the above-described removal of the SOI layer shall alsoapply to other Examples below.

The configuration of each of the phase shifter 1901 and the phaseshifter 1902 in FIG. 20 is the same as in Example 1.

In the rib waveguide 2001, a P region 2002, an N region 2003, a boundary2006, an intrinsic region 2004, an intrinsic region 2005, a P− region2007, an N− region 2008, a P region 2009, an N+ region 2010, a P++region 2011 and an N++ region 2012 are formed. A traveling-waveelectrode 2015 and the P++ region 2011 are electrically connected toeach other through a via electrode 2013, and a traveling-wave electrode2016 and the N++ region 2012 are electrically connected to each otherthrough a via electrode 2014.

In the rib waveguide 2021, a P region 2022, an N region 2023, a boundary2026, an intrinsic region 2024, an intrinsic region 2025, a P− region2027, an N− region 2028, a P region 2029, an N+ region 2030, a P++region 2031 and an N++ region 2032 are formed. A traveling-waveelectrode 2035 and the P++ region 2031 are electrically connected toeach other through a via electrode 2033, and a traveling-wave electrode2036 and the N++ region 2032 are electrically connected to each otherthrough a via electrode 2034.

FIG. 21 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 2 on a vertical surface including an alternate longand short dash line DD′. A lower cladding 2138 is disposed on asubstrate 2137. A rib waveguide 2101 that serves as the core of thewaveguide in the phase shifter 1901 and a rib waveguide 2121 that servesas the core of the waveguide in the phase shifter 1902 are disposed onthe lower cladding 2138. An upper cladding 2139 is disposed on the ribwaveguides 2101 and 2121. The configuration of each of the phase shifter1901 and the phase shifter 1902 is the same as in Example 2.

In the rib waveguide 2101, a P region 2102, an N region 2103, a boundary2106, an intrinsic region 2105, an N− region 2108, an N+ region 2110, aP++ region 2111 and an N++ region 2112 are formed. A traveling-waveelectrode 2115 and the P++ region 2111 are electrically connected toeach other through a via electrode 2113, and a traveling-wave electrode2116 and the N++ region 2112 are electrically connected to each otherthrough a via electrode 2114.

In the rib waveguide 2121, a P region 2122, an N region 2123, a boundary2126, an intrinsic region 2125, an N− region 2128, an N+ region 2130, aP++ region 2131 and an N++ region 2132 are formed. A traveling-waveelectrode 2135 and the P++ region 2131 are electrically connected toeach other through a via electrode 2133, and a traveling-wave electrode2136 and the N++ region 2132 are electrically connected to each otherthrough a via electrode 2134.

In the above-described two configurations of the Example, since thephase shifter 1901 and the phase shifter 1902 are independent from eachother, electric crosstalk is reduced between both phase shifters, and ahigh extinction ratio or a high Q value is easily obtained.

Example 4

Another configuration of an optical waveguide device functioning as theMach-Zehnder (MZ) optical modulator illustrated in the block diagram ofFIG. 19 will be described using the optical waveguide device describedin Example 1 or 2.

FIG. 22 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 1 on a vertical surface including an alternate longand short dash line DD′. A lower cladding 2238 is disposed on asubstrate 2237. A rib waveguide 2201 that serves as the core of thewaveguide in the phase shifter 1901 and a rib waveguide 2221 that servesas the core of the waveguide in the phase shifter 1902 are disposed onthe lower cladding 2238. An upper cladding 2239 is disposed on the ribwaveguides 2201 and 2221.

In the rib waveguide 2201, a P region 2202, an N region 2203, a boundary2206, an intrinsic region 2204, an intrinsic region 2205, a P− region2207, an N− region 2208, a P region 2209, an N+ region 2210, a P++region 2211 and an N++ region 2212 are formed. A traveling-waveelectrode 2215 and the P++ region 2211 are electrically connected toeach other through a via electrode 2213, and a traveling-wave electrode2216 and the N++ region 2212 are electrically connected to each otherthrough a via electrode 2214.

In the rib waveguide 2221, a P region 2222, an N region 2223, a boundary2226, an intrinsic region 2224, an intrinsic region 2225, a P− region2227, an N− region 2228, a P region 2229, an N+ region 2230, a P++region 2231 and an N++ region 2232 are formed. A traveling-waveelectrode 2216 and the P++ region 2231 are electrically connected toeach other through a via electrode 2233, and a traveling-wave electrode2236 and the N++ region 2232 are electrically connected to each otherthrough a via electrode 2234.

FIG. 23 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 2 on a vertical surface including an alternate longand short dash line DD′. A lower cladding 2338 is disposed on asubstrate 2337. A rib waveguide 2301 that serves as the core of thewaveguide in the phase shifter 1901 and a rib waveguide 2321 that servesas the core of the waveguide in the phase shifter 1902 are disposed onthe lower cladding 2338. An upper cladding 2339 is disposed on the ribwaveguides 2301 and 2321.

In the rib waveguide 2301, a P region 2302, an N region 2303, a boundary2306, an intrinsic region 2305, an N− region 2308, an N+ region 2310, aP++ region 2311 and an N++ region 2312 are formed. A traveling-waveelectrode 2315 and the P++ region 2311 are electrically connected toeach other through a via electrode 2313, and a traveling-wave electrode2316 and the N++ region 2312 are electrically connected to each otherthrough a via electrode 2314.

In the rib waveguide 2321, a P region 2322, an N region 2323, a boundary2326, an intrinsic region 2325, an N− region 2328, an N+ region 2330, aP++ region 2331 and an N++ region 2332 are formed. A traveling-waveelectrode 2316 and the P++ region 2331 are electrically connected toeach other through a via electrode 2333, and a traveling-wave electrode2336 and the N++ region 2332 are electrically connected to each otherthrough a via electrode 2334.

In the above-described two configurations of the Example, since the N++region 2212 or 2312 in the phase shifter 1901 and the P++ region 2231 or2331 in the phase shifter 1902 are electrically connected to each otherthrough the traveling-wave electrode 2216 or 2316 respectively, and asimple configuration of the traveling-wave electrodes is obtained, it ispossible to provide a small-footprint MZ optical modulator. In addition,it is possible to drive the MZ optical modulator in a push-pull schemeby applying a high-frequency electric signal to the traveling-waveelectrode 2216 or 2316. Therefore, zero-chirp modulation using a singlehigh-frequency signal source becomes possible, and it is possible tosimplify a high-frequency electric driving circuit.

Example 5

The other configuration of an optical waveguide device functioning asthe Mach-Zehnder (MZ) optical modulator illustrated in the block diagramof FIG. 19 will be described using the optical waveguide devicedescribed in Example 1 or 2.

FIG. 24 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 1 on a vertical surface including an alternate longand short dash line DD′. A lower cladding 2438 is disposed on asubstrate 2437. A rib waveguide 2401 that serves as the core of thewaveguide in the phase shifter 1901 and a rib waveguide 2421 that servesas the core of the waveguide in the phase shifter 1902 are disposed onthe lower cladding 2438. An upper cladding 2439 is disposed on the ribwaveguides 2401 and 2421.

In the rib waveguide 2401, a P region 2402, an N region 2403, a boundary2406, an intrinsic region 2404, an intrinsic region 2405, a P− region2407, an N− region 2408, a P region 2409, an N+ region 2410, a P++region 2411 and an N++ region 2412 are formed. A traveling-waveelectrode 2415 and the P++ region 2411 are electrically connected toeach other through a via electrode 2413, and a traveling-wave electrode2416 and the N++ region 2412 are electrically connected to each otherthrough a via electrode 2414.

In the rib waveguide 2421, a P region 2422, an N region 2423, a boundary2426, an intrinsic region 2424, an intrinsic region 2425, a P− region2427, an N− region 2428, a P region 2429, an N+ region 2430, a P++region 2431 and an N++ region 2432 are formed. A traveling-waveelectrode 2416 and the N++ region 2432 are electrically connected toeach other through a via electrode 2433, and a traveling-wave electrode2436 and the P++ region 2431 are electrically connected to each otherthrough a via electrode 2434.

In the phase shifter 1902, the disposition of the respectivedopant-distributed regions is reversed around a vertical axis along theboundary 2426 with respect to the phase shifter 1901.

FIG. 25 illustrates a schematic cross-sectional view of the phaseshifters 1901 and 1902 configured using the optical waveguide devicedescribed in Example 2 on a vertical surface including an alternate longand short dash line DD′. A lower cladding 2538 is disposed on asubstrate 2537. A rib waveguide 2501 that serves as the core of thewaveguide in the phase shifter 1901 and a rib waveguide 2521 that servesas the core of the waveguide in the phase shifter 1902 are disposed onthe lower cladding 2538. An upper cladding 2539 is disposed on the ribwaveguides 2501 and 2521.

In the rib waveguide 2501, a P region 2502, an N region 2503, a boundary2506, an intrinsic region 2505, an N− region 2508, an N+ region 2510, aP++ region 2511 and an N++ region 2512 are formed. A traveling-waveelectrode 2515 and the P++ region 2511 are electrically connected toeach other through a via electrode 2513, and a traveling-wave electrode2516 and the N++ region 2512 are electrically connected to each otherthrough a via electrode 2514.

In the rib waveguide 2521, a P region 2522, an N region 2523, a boundary2526, an intrinsic region 2525, an N− region 2528, an N+ region 2530, aP++ region 2531 and an N++ region 2532 are formed. A traveling-waveelectrode 2516 and the N++ region 2532 are electrically connected toeach other through a via electrode 2533, and a traveling-wave electrode2536 and the P++ region 2531 are electrically connected to each otherthrough a via electrode 2534.

In the phase shifter 1902, the disposition of the respectivedopant-distributed regions is reversed around a vertical axis along theboundary 2526 with respect to the phase shifter 1901.

In the above-described two configurations of the Example, since the N++region 2412 or 2512 in the phase shifter 1901 and the N++ region 2432 or2532 in the phase shifter 1902 are electrically connected to each otherthrough the traveling-wave electrode 2416 or 2516 respectively, and asimple configuration of the traveling-wave electrodes is obtained, it ispossible to provide a small-footprint MZ optical modulator. In addition,since the two doped regions (the N++ region 2412 or 2512 and the N++region 2432 or 2532) connected through the traveling-wave electrode havethe same electric conductivity, it is possible to suppress the drift ofthe DC potential of the traveling-wave electrode 2416 or 2516, and toprovide a stably operating MZ optical modulator.

Thus far, the invention has been described based on the preferredembodiments, but the invention is not limited to the above-describedembodiments, and a variety of modifications are permitted within thescope of the purpose of the invention.

Examples 1 and 2 describe examples in which the low conductive region isarranged between the dope region contiguous to the PN junction and theside wall of the rib, but the low conductive region having lowerelectric conductivity than the P region may be arranged so as to becontiguous to the side wall of the rib which is closer to the P regionthan N region. Also, the low conductive region having lower electricconductivity than the N region may be arranged so as to be contiguous tothe side wall of the rib which is closer to the N region than P region.

Examples 1 and 2 describe examples in which a doped region having alower carrier density than that of the doped region contiguous to the PNjunction is disposed immediately below the low conductive region(especially, the intrinsic region), but it is also possible to extendthe doped region contiguous to the PN junction (a doped region havingsubstantially the same carrier density as the above-described dopedregion) up to immediately below the low conductive region (especially,the intrinsic region), and furthermore, to extend the doped regioncontiguous to the PN junction up to a part of the slab in which the ribis not present on the slab.

Example 2 describes an example in which a low conductive region(especially, an intrinsic region) is disposed between the PN junctionand the side wall of the rib on the side having a P-type electricconductivity; however, conversely, it is also possible to dispose a lowconductive region (especially, an intrinsic region) between the PNjunction and the side wall of the rib on the side having an N-typeelectric conductivity.

In the above-described description, a MZ optical modulator having twoparallel rib waveguides (Example 1) provided with low conductive regions(especially, intrinsic regions) on both sides of the rib in the widthdirection and a MZ optical modulator having two parallel rib waveguides(Example 2) provided with a low conductive region (especially, anintrinsic region) only on one side of the rib in the width directionhave been described as the MZ optical modulators of Examples 3 to 5.However, it is also possible to dispose in parallel a rib waveguide(Example 1) provided with low conductive regions (especially, intrinsicregions) on both sides of the rib in the width direction and a ribwaveguide (Example 2) provided with a low conductive region (especially,an intrinsic region) only on one side of the rib in the width directionin a single MZ optical modulator.

The invention can be used as an optical waveguide device that enableshigh-speed refractive index modulation with low optical loss and lowdriving voltage.

What is claimed is:
 1. An optical waveguide device comprising: asubstrate; a lower cladding disposed on the substrate; a rib waveguideincluding a slab disposed on the lower cladding and a single ribdisposed on the slab contiguous to the slab; and an upper claddingdisposed on the rib waveguide, wherein the rib waveguide includes afirst doped region having a first electric conductivity exhibiting aP-type electric conductivity across the rib and the slab and a seconddoped region being contiguous to the first doped region and having asecond electric conductivity exhibiting an N-type electric conductivityacross the rib and the slab, a boundary between the first doped regionand the second doped region provides a PN junction formed in a directionperpendicular to a surface of the substrate and is disposed in acorrugated line in a propagating direction of guided light in the ribwaveguide in a plan view of the substrate, and the rib waveguideincludes at least one of a first low conductive region being contiguousto an opposite side of the second doped region in the rib and exhibitinglower electric conductivity than the second doped region and a secondlow conductive region being contiguous to an opposite side of the firstdoped region in the rib and exhibiting lower electric conductivity thanthe first doped region, wherein, in a case that the rib waveguideincludes the first low conductive region, a third doped region beingcontiguous to the second doped region and the first low conductiveregion and having the second electric conductivity is disposed in aregion immediately below the first low conductive region on the slab, afourth doped region having the second electric conductivity is disposedcontiguous to the third doped region in a part of the slab in which therib is not present on the slab, a carrier density in the first lowconductive region is lower than a carrier density in the third dopedregion, a carrier density in the third doped region is lower than acarrier density in the second doped region, and a carrier density in thefourth doped region is equal to or higher than the carrier density inthe second doped region, and in a case that the rib waveguide includesthe second low conductive region, a seventh doped region beingcontiguous to the first doped region and the first low conductive regionand having the first electric conductivity is disposed in a regionimmediately below the second low conductive region of the slab, aneighth doped region having the first electric conductivity is disposedcontiguous to the seventh doped region in a part of the slab in whichthe rib is not present on the slab, a carrier density in the first lowconductive region is lower than a carrier density in the third dopedregion, a carrier density in the seventh doped region is lower than thecarrier density in the first doped region, and a carrier density in theeighth doped region is equal to or higher than the carrier density inthe first doped region.
 2. The optical waveguide device according toclaim 1, wherein at least one of the first low conductive region and thesecond low conductive region is an intrinsic region.
 3. The opticalwaveguide device according to claim 1, wherein, in a case that the ribwaveguide includes the first low conductive region, a width of thesecond doped region is substantially constant in a propagating directionof the guided light, and in a case that the rib waveguide includes thesecond low conductive region, a width of the first doped region issubstantially constant in the propagating direction of the guided light.4. An optical waveguide device comprising: a substrate: a lower claddingdisposed on the substrate; a rib waveguide including a slab disposed onthe lower cladding and a single rib disposed on the slab contiguous tothe slab; and an upper cladding disposed on the rib waveguide, whereinthe rib waveguide includes a first doped region having a first electricconductivity exhibiting a P-type electric conductivity across the riband the slab and a second doped region being contiguous to the firstdoped region and having a second electric conductivity exhibiting anN-type electric conductivity across the rib and the slab, a boundarybetween the first doped region and the second doped region provides a PNjunction formed in a direction perpendicular to a surface of thesubstrate and is disposed in a corrugated line in a propagatingdirection of guided light in the rib waveguide in a plan view of thesubstrate, the rib waveguide includes at least one of a first lowconductive region being contiguous to an opposite side of the seconddoped region in the rib and exhibiting lower electric conductivity thanthe second doped region and a second low conductive region beingcontiguous to an opposite side of the first doped region in the rib andexhibiting lower electric conductivity than the first doped region,wherein, in a case that the rib waveguide does not include the secondlow conductive region, the first doped region is extended up to a partof the slab in which the rib is not present on the slab in the same sidewith the first doped region with respect to the boundary, and in a casethat the rib waveguide does not include the first low conductive region,the second doped region is extended up to a part of the slab in whichthe rib is not present on the slab in the same side with the seconddoped region with respect to the boundary.
 5. The optical waveguidedevice according to claim 1, further comprising: a first metal electrodedisposed on the upper cladding, wherein a fifth doped region having thesecond electric conductivity is disposed in the part of the slab inwhich the rib is not present on the slab in the same side with thesecond doped region with respect to the boundary, and the fifth dopedregion and the first metal electrode are connected to each other througha first through-hole via.
 6. The optical waveguide device according toclaim 1, further comprising: a second metal electrode disposed on theupper cladding, wherein a sixth doped region having the first electricconductivity is disposed in the part of the slab in which the rib is notpresent on the slab in the same side with the first doped region withrespect to the boundary, and the sixth doped region and the second metalelectrode are connected to each other through a second through-hole via.7. The optical waveguide device according to claim 1, wherein the ribwaveguide has a first rib waveguide and a second rib waveguide, and thefirst rib waveguide and the second rib waveguide are disposed parallelalong a width direction of the optical waveguide device.
 8. The opticalwaveguide device according to claim 7, wherein a part of the slab in thefirst rib waveguide closer to the second rib waveguide than the rib inthe first rib waveguide is connected to a third metal electrode disposedon the upper cladding through a third through-hole via, and a part ofthe slab in the second rib waveguide closer to the first rib waveguidethan the rib in the second rib waveguide are connected to a fourth metalelectrode disposed on the upper cladding through a fourth through-holevia.
 9. The optical waveguide device according to claim 7, wherein apart of the slab in the first rib waveguide closer to the second ribwaveguide than the rib in the first rib waveguide and a part of the slabin the second rib waveguide closer to the first rib waveguide than therib in the second rib waveguide are connected electrically to a commonfifth metal electrode disposed on the upper cladding through a thirdthrough-hole via and a fourth through-hole via respectively.
 10. Amethod for manufacturing an optical waveguide device with: a substrate;a lower cladding disposed on the substrate; a rib waveguide including aslab disposed on the lower cladding and a single rib disposed on theslab contiguous to the slab; and an upper cladding disposed on the ribwaveguide, wherein the rib waveguide includes a first doped regionhaving a first electric conductivity exhibiting a P-type electricconductivity across the rib and the slab and a second doped region beingcontiguous to the first doped region and having a second electricconductivity exhibiting an N-type electric conductivity across the riband the slab, a boundary between the first doped region and the seconddoped region provides a PN junction formed in a direction perpendicularto a surface of the substrate and is disposed in a corrugated line in apropagating direction of guided light in the rib waveguide in a planview of the substrate, and the rib waveguide includes at least one of afirst low conductive region being contiguous to an opposite side of thesecond doped region in the rib and exhibiting lower electricconductivity than the second doped region and a second low conductiveregion being contiguous to an opposite side of the first doped region inthe rib and exhibiting lower electric conductivity than the first dopedregion, said manufacturing method comprising: a resist production stepof, in a case that the rib waveguide includes the first low conductiveregion but does not include the second low conductive region, forming afirst resist having a resist side wall disposed in a corrugated shape inthe propagating direction of the guided light in the rib waveguide on ahorizontal surface in a location serving as a boundary between the firstlow conductive region and the second doped region, covering a regionserving as the second doped region, and exposing a region serving as thefirst low conductive region, in a case in which the rib waveguideincludes the second low conductive region but does not include the firstlow conductive region, forming a second resist having a resist side walldisposed in a corrugated shape in the propagating direction of theguided light in the rib waveguide on a horizontal surface in a locationserving as a boundary between the second low conductive region and thefirst doped region, covering a region serving as the first doped region,and exposing a region serving as the second low conductive region, and,in a case in which the rib waveguide includes the first low conductiveregion and the second low conductive region, producing the first resistor the second resist; and a resist trimming step of trimming the firstresist or the second resist after the resist production step, therebyforming a resist having a resist side wall disposed in a corrugatedshape in the propagating direction of the guided light in the ribwaveguide on a horizontal surface in a location serving as the PNjunction on a plan view of the substrate.